Exhaust gas emitted from an internal combustion engine, such as a gasoline engine or a diesel engine, of an automobile, a motorcycle (also referred to as a saddle-type vehicle), etc., contains hazardous components such as NOx, HC, and CO. Three-way catalysts have conventionally been used to remove these hazardous components and render the exhaust gas harmless. Examples of three-way catalysts that have been used include mixtures including discretionary amounts of noble metals, such as Pt, Pd, and Rh, and alumina, ceria, zirconia, and/or composite oxides thereof.
As a part of exhaust gas regulation, the OBD-II regulation was introduced in the United States from around 1990, the regulation mandating the installation, onto a vehicle, of a system that: detects abnormalities, such as deterioration of an exhaust gas purification catalyst (hereinafter also referred to simply as “catalyst”); and, upon detection of such abnormalities by an on-board diagnostic (OBD) device, notifies the driver of the abnormality by lighting up a warning lamp on an instrument panel, and stores the failure code(s) which can be read out with a standardized scanning tool. Similar regulations have been introduced in Europe and Japan. Thus, there is a demand for an inexpensive, versatile system capable of detecting deterioration of a catalyst.
As disclosed in Patent Literatures 1 and 2, conventional catalyst deterioration detecting systems generally employ O2 sensors on both the upstream and downstream sides of a catalyst, and judge deterioration of the catalyst on the basis of fluctuations in the outputs of the O2 sensors. Patent Literatures 3 and 4, on the other hand, disclose techniques that do not employ O2 sensors but instead include temperature detection means on both the upstream and downstream sides of a catalyst in an exhaust passage of an internal combustion engine, and detect deterioration on the basis of a difference between the temperatures detected by both means.